A solder paste is a material that is capable of being applied to a substrate or surface in a specific pattern using screening or analogous methods which can subsequently undergo fusing to provide an electrical connection commonly referred to as a solder joint. Solder pastes generally comprise powdered metallic solders of various alloys such as tin-lead, tin-lead-silver, tin-lead-bismuth, tin-lead-antimony, tin-silver etc., contained in a vehicle including a flux. In addition a suspension medium, such as hydrogenated castor oil, may be present in the vehicle and combined with the flux to provide a solder paste of uniform texture.
In electronic circuitry, solder pastes are employed to secure electronic components to a circuit on a substrate such as plastic, ceramic, glass or the like. Circuits, similar to those used on printed circuit boards (PCBs) are produced on the substrates. The areas of the circuit to which electronic components are to be added are commonly referred to as pads. Generally in a typical surface mount electronics manufacturing process, a solder paste is screen or template printed onto the pads of the printed circuit board. Next, the electronic components are carefully positioned with their peripheral leads in contact with the solder paste-coated pads. The populated board is then passed through an oven to melt the solder in the paste and wet the pad and lead surfaces, thus achieving the solder joint.
Flux residues are generated by the reflowing of the solder paste. The flux residues are often corrosive and therefore must be removed from the circuit board. Typically, the removal of the flux residues is accomplished by washing the printed circuit board in a chlorinated-fluorocarbon (CFC) based solvent. Since CFC solvents present environmental hazards in their use and disposal, the removal of flux residues in addition to creating an extra step, may cause hazardous conditions in the workplace.
A further problem in generally performed surface mount electronic assembly in particular, fine-pitch electronic components, is that conventional solder paste often leads to electrical shorts between adjacent lead/pad connections. Traditionally, "anisotropic" or "Z-Axis" materials have been used to address this problem. Anisotropic materials interconnect electrically to metal surfaces by virtue of physical contacts created between said metal surfaces and conductive particle-to-particle contacts contained within the Z-Axis material to create an electrical continuity. These electrical contacts are maintained by fixing in place leads and pads and the particles in between them with a hardened resin and, optionally, holding the joints together with a dedicated, often permanent, fixture or clamp.
It would be advantageous in a surface mount electronic manufacturing process to have an anisotropic solder paste which does not produce corrosive flux residues when reflowed, and which minimizes the formation of electrical shorts between fine-pitch lead/pad joints during reflow soldering.